Sequential Circuits Pro-One

Jun 13, 2016

The Sequential Circuits Pro-One Synthesizer

After the success of the legendary Sequential Circuits Prophet 5 polyphonic synthesizer, Sequential released a synth that was in many ways a monophonic version of the Prophet 5. While the build quality of the Pro-One was not in the league of it’s polyphonic sibling it certainly matched the Prophet 5 sonically. It is an intense, punchy little synth that is, according to the Pro-One owners manual identical to one voice of the Prophet 5. In some ways however it manages to exceed the power of a single Prophet 5 voice.

Placed beside a Revision 3 Prophet 5 the Pro-One sounds even punchier and more immediate that the Prophet’s individual voices. Perhaps this is due to the biggest difference between the architecture of the two machines – The Pro-One does not have memories/presets and therefore does away with a whole layer of computer circuitry between the controls and the synth engine that synths such as the Prophet require in order to be able to store patches. The pots and switches on the Pro-One’s front panel are a part of the actual synthesizer circuits. The Prophet’s controls are connected to the computer. The other factor that caused differences to the final sound is in the setup – the component values and configurations that connect the synth modules aren’t exactly the same.

Pro-One oscillator section

The Pro-One is functionally a Prophet 5 voice. Two Curtis Electromusic CEM 3340 Oscillator IC’s, a CEM3320 filter, a pair of CEM3310 envelope generator IC’s and a CEM 3340 VCO (underutilized) as an LFO. It also used the same CA3280 transconductance op-amp IC for it’s VCA as the Prophet. Shown here are the two CEM3340 VCO’s one above the other on the left side of the photo.

An additional CEM3340 takes care of the LFO duties, this can be seen on the bottom RHS. The other IC that is visible on the top right is an op-amp that sums and buffers the octave switching for both VCO’s.

Note also the switches and pots – Compared to current synths they are large and robust indeed.

Pro-One computer section

The Pro-One synth circuitry is pure analog, it does however differ from other mono-synths in that the key control voltages, triggers and gates are generated by an internal microcomputer. The computer reads the keys of the keyboard, the Mode, Sequence and Arpeggio switches. When the computer detects a key being pressed it sends data to the DAC which converts the data to an analog control voltage for the synth circuits.

In the image you can see the Intel 8021 microcomputer IC at the bottom. It contains the CPU, RAM and ROM all on one chip. The little gold chip above it is the DAC. Why put a microcomputer inside the Pro-One in the first place? The answer is that without it there would be no sequencer or arpeggiator.

Pro-One 8 bit DAC

In this photo you can see the Pro-One’s digital to analog converter. It is an Analog Devices AD558JD. This chip is an 8 bit A/D converter. It can take an 8 bit data bus and convert it into an analog voltage that is compatible with the CV’s that are found inside analog synths. For accuracy only the 6 most significant bits are used. It is scaled to provide 83mV per step which is 1/12 of a Volt, adding up to exactly 1 Volt for the 12 notes in an octave, hence the term “1V/octave”. There are 12 spare steps left in the DAC which are used for the sequencer transpose function.

Pro-One power supply and ADSR

In this image you can see the four voltage regulators that provide the stable power supply rails for the Pro-One. These little three pin devices are quite extraordinary – they revolutionised power supply design when they were released and they are a staple to this very day. Each chip provides a clean output at the chosen voltage even if the input voltage is noisy or somewhat unstable. They are reliable and replace a whole lot of bulky circuitry and are used in the majority of vintage synthesizers. At the bottom centre you can see a CEM 3310 envelope generator IC that produces the envelope for the VCA.

Of special interest in the power supply photo is the clear damage to the circuit board at the top right of the image. On the earlier versions of the Pro-One the mains power transformer and switch was mounted on the main circuit board, a very unsatisfactory design feature that was rectified on later models where the transformer was moved to the bottom of the case. The circuit tracks were too close together and while this was bad enough at 110V it became worse at 240V in Europe and Australia. Risky for service techs working on the synth circuits and occasionally the electricity arced over and here you can see the result. Copper tracks burned off the circuit board and in this case a hole blown through the side of the VCF Keyboard amount pot.

The Pro-One rear panel. Neat and functional.

A closeup of a Pro-One VCO and CV summing resistors

The Pro-One circuit board. A one board synthesizer.

The Sequential Circuits Pro-One. A beautiful and iconic synthesizer.

Pro-One pitch and modulation wheels.

The modulation section, the Prophet lineage is clear.

An option that never happened?

Sequential and E-mu often included mandalas and other imagery on their circuit boards.

The architecture of the Pro-One matches that of a Prophet 5 all the way down to the wheel mod and LFO control routing. The comparison ends at the physical build of the machine though. A plastic top panel, the pots (knobs) that are firmly bolted to the P5’s metal chassis are loose and wobbly on the Pro-One – no nuts and bolts here, it’s a budget build but nonetheless it’s a pretty synth and nicely laid out and it has the added bonus of a sequencer and arpeggiator.

It has very stable tuning and it’s quite easy for the musician to scale the VCO’s via the little holes on the LHS of the top panel. The tuning procedure is documented in the owners manual. With the aid of a simple guitar tuner and a small screw driver the Pro-One owner can take care of tuning drift at home every year or so as needed. No need for a visit to the tech shop! The CEM 3340 VCO’s have their own thermal compensation so the synth handles ambient temperature variation quite gracefully too.

Pro-One buying guide and common issues

As with any vintage synth the condition of the exterior tells a story and usually reflects the condition of the interior. It’s worth paying a buyers premium for a good example of the machine of you can find one, if you ever choose to sell it in the future you will see a return. When trying out the synth the golden rule is to test EVERYTHING. Take your time, read the manual if you need to before you go to inspect the synth. Look for the obvious – Cosmetics, pots not excessively wobbly and not scratchy, smooth, even key action, jacks clean and not intermittent, bend and mod wheels smooth and working. test every pot and switch, especially check all of the waveforms on the VCO’s and LFO, it’s not unheard of for a CEM3340 VCO to lose one of it’s waveforms while the others still function.

The Pro-One uses a Pratt-Read keyboard mechanism in common with many American synthesizers. After so many years the rubber bushings dry out and the keyboard becomes noisy and uneven. This is a common problem and easy to repair. These bushings have been reproduced by several enterprising people and are available for a reasonable cost. They can be replaced by anyone that is reasonably handy and careful, the suppliers provide directions on their respective web sites. We stress that if you attempt the job yourself that you must wear safety glasses and be careful not to lose the little springs that you will need to remove.